1. Field of the Invention
The present invention relates to a forward-looking sonar for displaying echoes of objects ahead of a surface vessel by use of transmitted and reflected ultrasonic waves (hereinafter referred to as acoustic waves). More particularly, the invention pertains to a forward-looking sonar capable of measuring and visually presenting the distance and direction of any of such navigational hazards as shallows and underwater obstacles ahead of a surface vessel.
2. Description of the Related Art
A forward-looking sonar for measuring and visually presenting the distance and direction of any of such navigational hazards as shallows, reefs, drifting surface objects, underwater obstacles and a sea bottom ahead of a surface vessel is installed as shown in FIG. 27. A transducer of the forward-looking sonar is typically installed on the hull of the vessel as illustrated. By using an acoustic sounding beam formed by the transducer, the forward-looking sonar searches for any of the aforementioned underwater obstacles present within a fan-shaped sounding area from the surface of the sea to the bottom (90 degrees downward from the surface).
There exist conventionally known examples of this kind of forward-looking sonars.
For example, U.S. Pat. No. 5,675,552 discloses a sonar apparatus using a phased array technique including a transducer for transmitting and receiving acoustic waves, the transducer having multiple elements which are arranged in line to constitute a linear array. Phase differences among transmit signals fed into the individual transducer elements and phase differences among receive signals obtained by the individual transducer elements are so controlled as to successively steer an acoustic transmitting/receiving beam from the surface to the bottom over a 90-degree sector area to determine the direction of a return echo from an underwater object. The sonar apparatus determines the distance to the object based on time needed for receiving the return echo after transmission. FIG. 28 is a diagram showing how the acoustic transmitting/receiving beam formed by the linear array transducer is successively steered.
U.S. Pat. No. 5,530,680 gives another prior art example. An echo sounding apparatus described in this U.S. patent includes a transducer for transmitting and receiving acoustic waves, the transducer having multiple elements which are arranged in line to constitute a linear array. The echo sounding apparatus determines the direction of a return echo from an underwater object by a split-beam method and the distance to the object based on time needed for receiving the return echo after transmission.
According to the method of U.S. Pat. No. 5,530,680, the echo sounding apparatus transmits the acoustic waves in a 90-degree sector area by one transmission from a single element and receives return echoes from an underwater object by using two paired elements as illustrated in FIG. 29. The echo sounding apparatus determines the direction of the object from a phase difference between the echoes received by the paired elements and the distance to the object based on time needed for receiving the echoes from the object after transmission.
The aforementioned split-beam method is a method of determining the direction of a target from a phase difference between echoes from the target detected by two receiving beams directed toward the target. Referring to FIG. 30, if echoes from a target are received by two elements R, L from a direction θ, a phase difference φ between the echoes received by the two elements R, L varies with the direction of the target, so that it is possible to know the direction of the target from the phase difference φ. Given a wavelength λ of the acoustic waves and an element-to-element distance d, the phase difference φ is calculated by equation (1) below:
                    ϕ        =                                                            2                ⁢                π                ⁢                                                                  ⁢                d                            λ                        ·            sin                    ⁢                                          ⁢          θ          ⁢                                          ⁢                      (            rad            )                                              (        1        )            
From equation (1) above, the direction θ is obtained by equation (2) below within beam angles of the individual elements R, L:
                    θ        =                              sin                          -              1                                ⁢                      ϕλ                          2              ⁢              π              ⁢                                                          ⁢              d                                                          (        2        )            
Equation (2) above indicates that the direction θ of the target is linearly related to the phase difference φ as shown in FIG. 31. Since the wavelength λ and the element-to-element distance d have fixed values, the accuracy of the direction θ obtained by equation (2) is determined by measuring accuracy of the phase difference φ.
The aforementioned prior art arrangements however have their respective drawbacks.
Specifically, one drawback of the arrangement of U.S. Pat. No. 5,675,552 is that the sonar apparatus requires considerable time for searching for obstacles. Since the transducer transmits acoustic waves and receives return echoes in one direction to another in a step-by-step sequence, it normally takes a good deal of time to obtain an underwater cross-sectional image within a 90-degree fan-shaped search area ahead of a vessel. If the 90-degree area is to be searched by successively steering the transmitting/receiving beam in 3-degree steps, for example, it would be necessary to repeatedly perform transmit/receive cycles 30 times.
Another drawback of the arrangement of U.S. Pat. No. 5,675,552 is poor direction measuring accuracy. The sonar apparatus displays echoes detected within a beam angle of the transmitting/receiving beam and, therefore, the direction measuring accuracy is determined by the beam angle. Particularly when the sonar apparatus is operated on a larger range scale for detecting obstacles at greater distances, a bottom echo displayed on-screen becomes larger (displayed over a longer range) as shown in FIG. 32, so that a range at which targets can be distinguished from the bottom is limited on larger range scales. Although this drawback can be more or less overcome by reducing the beam angle of the transducer, it is generally necessary to increase array length by using a larger number of elements to constitute a longer linear array transducer. This approach results in an increase in physical size and manufacturing cost of the transducer, also imposing a problem related to transducer installation.
The aforementioned arrangement of U.S. Pat. No. 5,530,680 has a drawback in that the echo sounding apparatus has a limited sounding range. Since the echo sounding apparatus transmits acoustic waves in the 90-degree sector area by using a single element, transmitting sound pressure (or source level) is low. Sensitivity to return echoes is also low and the sounding range is limited since the return echoes are received by each of the two paired elements alone.
Another drawback of the arrangement of U.S. Pat. No. 5,530,680 is losses, or dropouts, of return echo data. In the split-beam method used in the arrangement of U.S. Pat. No. 5,530,680, the echo sounding apparatus can not distinguish echoes from two or more equidistant targets located in different directions (just like echoes from equidistant points P and Q shown in FIG. 9A which will be later described) as these echoes arrive at the same time. This causes a problem of echo data dropout.